E-Man,
Flashing headlights in the U.S. can mean any number of things. In daylight, when oncoming traffic starts flashing headlights it means either a speed trap ahead or a roadblock ahead. (saved my ass many times!) Therefore if you have just passed a speed trap or went thru a roadblock, it is a common courtesy to signal other drivers of the pork ahead. Now if there is an actual hazard in the road, a wreck or debris or something like that, it's best to reduce speed to a crawl even after passing it or even stop and signal oncoming traffic a little more urgently.
At nighttime, turning headlights on and off (not the high-beams) is how you tell another driver in the oncoming lane to turn their headlights on. Also at nighttime, when someone in the oncoming lane has their high-beams on you flash your high beams a couple of times to remind them to dim their headlights. If they don't, then leave your high beams on and flip them off as you pass and mutter a few curses against SUV's (it's always an SUV, you know?) Or you could do like I used to do when I delivered pizzas. I had this super-mega-powerful spotlight for finding house numbers. When the flash-the-high-beams thing didn't work, I pointed the spotlight at the jerk. That always worked.
Also when on the interstate over here (freeway, or I think over there they're called motorways) when an 18-wheeler (a lorry?) wants to merge in your lane in front of you, you flash your headlights to signal that it's clear. If the trucker is one of the nice ones, he'll (she'll) say "thank you" by flashing the marker lights on the trailer. Always be nice to the big trucks no matter how big a pain in the ass they are. They're bigger than you and can squash you like a bug and keep right on without a care.
But over here, you don't flash your headlights to tell another motorist to "go ahead". If you want to give them the right of way or whatever you make eye contact and gesture to them (no not that gesture!) to go ahead, waving them to move over or take off or whatever. Of course it is expected of polite drivers that when someone does yield to you in that manner that you give them a big smile and wave and mouth "thank you".
Now as far as flashing your headlights to pass (overtake) another driver. Sometimes it works, sometimes it doesn't. I'm focusing on the interstate (motorway) here. The slowpoke could be a 100-year-old tourist on their way to Florida (just drive on I-75 and you'll understand), a dumbass soccer mom in a Ford Excursion yakking away on her cell-phone, or an asshole from Houston County (again, just drive on I-75 and you'll understand), none of which will move over for you. So don't keep flashing. Just wait for the opportunity and pass on the right or in the breakdown lane.
Mike.
p.s.
When I said "just drive on I-75 and you'll understand" I guess I do need to explain for some of you. I bet Francois will know exactly what I'm talking about. All the old folks from up north take I-75 to get to Florida. Now I'll admit that a few do indeed drive well but far too many of them are a real hazard. They get out in the middle or left lane, but never the right-hand "slow lane", and go slower than traffic, oblivious to everyone else around them. Usually with those gargantuan wrap-around sunglasses on and their noses pressed against the top of the steering wheel. And just to be extra annoying, they leave their left turn signal on for hours on end.
Now as far as the "self-absorbed assholes from Houston County", well that's a mystery I myself have yet to unravel and I've lived here all my life. Houston County is where I grew up, home to Robins Air Force Base and the famous town of Perry I've written about in the past. Bunch of rednecks down there. Bunch of rude arrogant hateful selfish rednecks. (Ravyn lived in Warner Robins, she knows!) On the Middle Georgia section of I-75 if you happen to be stuck behind a slow-moving vehicle in the middle lane, 7 times out of 10 it has a Houston County tag. Never the left lane, just the middle lane. (just 3 lanes in each direction, and the signs clearly state "slower traffic keep right") I'm not making this up y'all and I'm not exaggerating, not one bit!!!! I've driven from Houston County to Macon and back too many times in the last many years and on all those trips I've taken notice of the tags. Once in a great while, you'll see someone with a Houston County tag driving along with traffic or even faster. But for the most part, and I mean 98% of the time, they get in the middle lane and creep along at their own slow pace. They won't move over or even acknowledge other drivers, even the 18-wheelers bearing down on them. Unless it's at night and in light traffic. Then they like to see just how asshole-ish they can be. Once you pass, they speed up just a little and move so that their headlights shine in your sideview mirror for a while, then speed up and get back in front of you only to slow down again. There's been times on the 30 mile drive between Macon and Perry when I've "played leapfrog" with the same car almost the whole distance.